Road diets occur when lane widths and/or number of lanes are adjusted to promote a slower vehicle speed and or accommodate other modes of traffic in the right-of-way including bicycles, pedestrians, and transit. Lane width is the distance between the identified spaces where vehicles travel along the roadway. On arterial streets lane widths are often identified with striping. On residential streets, the travel lane is often not striped or marked.

The number and width of the travel lanes on the street affects the pedestrian crossing width, crossing times, the speed of cars, parking, comfort of the street for pedestrians, bike lanes, and the turning movements at the intersections. A road diet is a technique that can be used to achieve traffic calming.